This invention relates to the field of aircraft cabin environmental control systems and in particular to the use of an electrical resistance heater to provide heat for an aircraft cabin.
An environmental control system for the passenger cabin of an aircraft usually acts to maintain the ambient air in the cabin at both a comfortable temperature and pressure. Some smaller planes, which are not designed to reach very high altitudes, do not have any provision for pressurization of the cabin. However, nearly all planes include some provision for both warming up and cooling down the temperature of the air within the cabin.
Jet aircraft and turbo-prop aircraft usually obtain heated air for cabin temperature purposes as a by-product of the compression of the air used to drive the jet or turbo-prop engines. The compression itself acts to heat the air. This heated air is usually provided for use in adjusting cabin temperature by using appropriate ducting. Thus the big commercial airline jets and turbo-prop planes usually do not use a separate heat source to maintain a comfortable cabin temperature.
Single engine propeller airplanes usually take the heat needed to maintain a comfortable cabin temperature directly from a heat exchanger located around the manifold of the engine. This is because the engines on these planes ar usually mounted at the nose of the plane directly in front of the passenger cabin. Thus where heat is required because the outside air temperature (OAT) is too cold, the heat is provided by passing outside cold air directly over the manifold of the engine and then into the cabin. The ducting for the heat transfer from the exhaust manifold of the engine to the cabin is fairly straight forward in these single engine planes.
In the case of a twin engine propeller, fixed wing aircraft the problem of how to provide heat to the interior of the cabin has been approached in two ways. One approach is to use the heat directly from one or both of the engines to heat the outside air. Because these engines are mounted on the wings it is necessary to provide ducts which extend through the wings to the cabin. Although the cabins of some twin engine planes are heated in this way, most are not. The reason for this may be that because of the small size of the planes it is very difficult, from a design point of view, to provide the necessary ducting through the wings of the plane. Additionally, because of the proximity of the ducting to the exhaust of the engines it is necessary to inspect the ducts for cracks on a regular basis to be sure that carbon monoxide is not being drawn into the cabin. The longer and more convoluted the ducting is, the more difficult such an inspection becomes.
In any event, the fact is that very few twin engine, propeller planes use heat obtained from the engines to heat the cabin. Instead most of these planes provide a separate fuel combustion chamber and exhaust port located in the nose of the plane. These combustion heaters are an attempt to duplicate the heat source which would otherwise be available at the nose of the plane if an engine was mounted in the nose. The idea is that it is easier to create an "exhaust manifold" in the nose of the plane than it is to provide the ducting through the wings necessary to utilize the heat from the exhaust manifolds on the engines.
In the case of unpressurized twin engine planes, cold air from outside the plane is passed through a heat exchanger in close proximity to the combustion heater to provide heated air for the cabin. In such planes the heated air is discharged form the unsealed cabin fairly rapidly allowing freshly heated air to quickly take its place.
In pressurized planes the cabin is sealed and is pressurized by compressors, run by the plane's engines. These compressors compress outside air and inject it into the cabin through the wings at a pressure somewhat higher than the desired pressure for the cabin. The desired pressure is maintained by the provision of a pressure relief valve at the rear of the cabin which continuously vents air to the atmosphere. The amount of fresh air flow obtained through the compressors is usually fairly small compared with that which flows through the unpressurized planes. However, the fresh air provided to the cabin in this manner is sufficient for breathing purposes. In some pressurized planes the pilot has the option of providing heat in the cabin by mixing outside air with recirculated cabin air or simply recirculating the cabin air only through the combustion heater.
The most widely used combustion chamber airplane heater is the Janitrol combustion heater. The Janitrol heater is formed of two concentric cylinders. The inner concentric cylinder is sealed at both ends to form a combustion can. The combustion can has an opening for admitting fuel directly from the airplane's fuel tank, along with the air needed to burn the fuel. In addition, an opening is provided for exhaust fumes to be ejected. A spark plug is provided for igniting the fuel and air mixture. When it is desired to heat the interior of the cabin, fuel and air are injected into the combustion can and ignited by the spark plug. An exhaust fan is provided to blow the exhaust gases out of the exhaust port after combustion.
The consequent explosions inside the combustion chamber generates a great deal of heat on the outer surface of the inner cylindrical can, just as the explosions within the engine cylinders create a great deal of heat on the outside surfaces of exhaust manifolds of aircraft engines. The outer concentric cylinder is spaced from the inner concentric cylindrical can to permit air to flow therebetween. Air from the outside and, in the case of some pressurized planes, air from the cabin, is forced through this annular air space and then into the cabin. As the air passes by the extremely hot surface of the inner combustion chamber the air is warmed up. This warm air, in turn, warms up the cabin.
The Janitrol combustion heater is a very effective heater. However, the combustion heater has a number of problems. One of the most obvious problems is that a separate combustion chamber, apart form the combustion chambers provided by the plane's engines, is provided in the nose of the plane. Every time the heater is turned on, fuel from the aircraft's fuel tank is pumped into the combustion chamber and ignited.
Although the combustion chamber of the Janitrol heater is intended to withstand the resulting explosions a great deal of stress is crated. For this reason the F.A.A. considers the combustion heater to be a potentially serious hazard and requires that all combustion heaters be inspected on a yearly basis. The inspection is intended to determine whether or not there are any cracks or defects in the combustion container. Because there is no ready access to the combustion container this inspection procedure is very expensive. Further, if the Janitrol heater must be reconditioned the expense can run mush higher.
In view of the dangerous nature of the Janitrol heater a number of people in the business refer to the heater as a "flying bomb". Further, a number of accidents have been traced to Janitrol combustion heaters. Malfunction of a combustion heater has been known, for example, to cause flames to shoot out of the exhaust port of the heater and around the cockpit of the plane.
As a result of the dangerous nature of these combustion heaters as well as the expense of complying with F.A.A. regulations, many pilots have simply disconnected the heaters. Although a cabin heater is not always required, disconnecting the heater is certainly not a satisfactory solution.